Some variable displacement axial piston hydraulic pump designs utilize the naturally occurring swivel torque as a means for controlling pump displacement. This eliminates the need for the conventional hydraulic actuators normally used for controlling swashplate position to change pump displacement, thereby reducing the size of the pump and increasing pump efficiency.
One of the problems associated with elimination of the hydraulic actuator is that the stabilizing effect resulting from the actuator bulk modulus is also eliminated causing the pump to become unstable. Upon analyzing the problem, it was found that piston inertia created from the oscillating displacement of the piston itself is responsible for driving variable displacement pumps unstable. In general, this inertia produces a torque on the swashplate that may be expressed as: ##EQU1## wherein: T=inertial torque on the swashplate
N=number of pistons PA1 M=piston mass PA1 r=piston pitch radius PA1 s=pump rotational speed PA1 a=swashplate angle.
The inertial torque described by this equation drives the pump unstable because of its positive sign.
Analysis has shown that making the torque value negative, or at least zero, would make the pump completely stable. Thus, it would be desirable to provide a piston that creates a neutral or negative inertial torque rather than a positive one. Further, since the object of eliminating the hydraulic actuator was to reduce the size of the pump, the new piston must not consume any more space than the present piston.